1. Field of the Invention
The invention relates to protective layers for metals or metal alloys that can be used at high temperatures and in aggressive gaseous, liquid and solid media. Stated more precisely, the present invention relates to a thermally sprayed gastight protective layer for metal substrates, especially those based on Fe, Ni, Al, Mg and/or Ti, wherein the spray powder for the purpose comprises at least two components, of which the first is a silicate mineral or rock and the second is a metal powder and/or a further silicate mineral or rock.
2. Description of the Related Art
Enamels are known as nonmetallic protective layers for various metals and alloys (see [1]: A. Petzold, H. Pöschmann, Enamel and Enameling Technology, Wiley-VCH; Edition 2 (1992)). These protective layers have good adhesion to the substrate and protect the metal base materials reliably against many aggressive media up to approximately 400° C. In industry, silicate glasses having a relatively low SiO2 content and a high alkaline oxide content are used as enamels for steels and cast iron (see [1]). Typical enamels for white enameling of steel sheet comprise a base and a cover enamel and have the following compositions:
Base enamelCover enamelSubstanceContent (%)SubstanceContent (%)SiO247-53SiO256Al2O34-6Al2O37B2O317-19B2O37Na2O + K2O15-18Na2O + K2O22.5TiO22-8CaO7CaO + MgORemainderF0.5
Special enamels for aluminum, copper alloys, stainless steels, titanium and other metals usually have even less SiO2 and more alkalis than do enamels for steel and cast iron.
A high alkali content influences the corrosion resistance of the silicate enamels to water and acids negatively, but is absolutely necessary for the enameling process: firstly to keep the melting temperature low and secondly to achieve a high coefficient of thermal expansion—adapted to the respective substrate. Because of the enameling method, enamels for steels must have a melting point (liquidus temperature TL) below 850° C., while that of enamels for aluminum must even be below 550° C. (see [1]). Low melting temperatures and high necessary coefficients of thermal expansion make it impossible to use enamels of known acid-resistant glasses, such as silica glass, borosilicate glasses, E-glass, acid-proof porcelain glazes and others.
Also known are ceramic layers of high-melting, corrosion-resistant materials, which are applied on metal substrates by means of thermal spraying (flame spraying, high-velocity oxygen-fuel flame spraying (HVOF), plasma spraying) or PVD and CVD methods. For example, yttrium-stabilized zirconium oxide (YSZ) can be applied by thermal spraying [UK 2100621 A; U.S. Pat. No. 4,377,371; WO 91/05888; U.S. Pat. No. 5,169,689] and also by PVD [U.S. Pat. No. 4,321,310; U.S. Pat. No. 4,321,311; U.S. Pat. No. 4,401,697; U.S. Pat. No. 4,405,659; WO 92/0598] on substrates of steel and of nickel-base alloys. In YSZ layers, any difference between the coefficients of thermal expansion of layer and substrate is compensated by a porous structure having a network of fissures. Because of this property, such layers are resistant to thermal shock. However, they do not guarantee protection against oxidation and corrosion and can be used merely as pure thermal insulation layers at temperatures up to 1200° C. A second important disadvantage of YSZ layers lies in weak adhesion to the substrate. Together with low mechanical strength (because of fissures and pores), this means poor erosion resistance.
Other known ceramic layers such as TiN, TiC, CrC, CrN, DLC, etc., which are formed by PVD/CVD methods, have low coefficients of thermal expansion and therefore cannot be used at high temperatures; in particular, the layer becomes detached when the temperature is raised, because a metal substrate expands much more than the layer. For this reason, these very thin layers with layer thicknesses of less than 5 μm are used mainly for wear and corrosion protection at room temperature.
Further protective layers employed as thermal insulation for high-temperature applications are known from DE 19852285 C1 and EP 1141437 B1. In contrast to YSZ, these glass-metal/ceramic layers are free of pores and fissures, and so are gastight. The adhesion to the metal substrate is also substantially better than in the case of YSZ layers, because the metal surface is wetted by the glass component of the layer. The cited layers of the class in question are also resistant to thermal shock, because the coefficients of thermal expansion of the layer, of a metal intermediate layer that may be present and of the substrate are approximately equal or are matched to one another. A metal content improves the mechanical properties of the layer. Matching of the coefficients of thermal expansion is possible by variation of the glass composition and/or of the metal-glass or ceramic/glass ratio.
These glass-metal/ceramic layers are used as thermal insulation layers for turbine blades. An advantage compared with YSZ layers lies in oxidation protection for the substrate by the gastight layer microstructure. Nevertheless, these layers are not suitable as corrosion-protection layers. For the glass-metal/ceramic layers according to the prior art, it has been necessary to choose alkali-containing glasses, in order to achieve the highest possible coefficient of thermal expansion in order to match that of the substrate. For use as thermal insulation layers, this is also not critical.
In contrast, it is the object of the present invention to provide a thermally sprayed and gastight protective layer of the class in question for metal substrates, especially those based on Fe, Ni, Al, Mg and/or Ti, as well as a method for production of the same, wherein the layer offers corrosion protection for the substrate, even at high temperatures.